The present invention relates to an opto-electronic method of analyzing distortion in a piece of shaped glass in an objective, quantitive way that is nonetheless representative of visual human assessment of such distortion.
Detection of defects in commercially produced flat glass has always been critical in the flat glass industry. High volume production of flat glass by the float process has required detection of defects very quickly, as the continuous ribbon of glass is moving past a given stationary point at a speed of hundreds of inches per minute. Human visual assessment and automated systems have both been used to detect glass defects in float glass production processes, with visual human assessment often being as reliable, or more reliable, than automated systems, due to the large number of variables which can affect a rapidly moving ribbon of glass.
Often, such float glass is further processed, for example, to shape a glass sheet or panel for use as a vehicle glazing. Glass shaping processes, typically, involve reheating the glass sheet or panel to, or near, its softening temperature, and then shaping the glass by, for example, gravity bending or press bending. Such heating and shaping operations have the potential to create new kinds of defects in the glass sheet or panel, for example optical distortion. As with the float glass production process defects, visual human assessment has most often been the most effective way of judging the acceptability of optical distortion in shaped glass sheets or panels. Visual human assessment is necessarily, however, subjective, and quantification of distortion by such assessment methods, unreliable. Efforts to date to develop an automated system of assessment of optical distortion in shaped glass, at least by utilization of reflected light have been, largely, unsuccessful, due as in the case of assessing defects in the case of float glass production, to the large number of variables which must be taken into account. It would be desirable to have an automated system to quantitatively assess optical distortion in shaped glass sheets or panels, by use of a method which closely approximates human visual assessment, that is, by viewing a reflected optical image.
Automated systems purported to detect and measure defects in shaped glass sheets or panels are the subject of many U.S. patents. For example:
U.S. Pat. No. 4,853,777 describes a system to quantitatively determine the short-term and long-term waviness of a smooth surface by impinging light radiation, namely laser light, onto the surface, detecting the resultant light image and mathematically processing the subject light image.
U.S. Pat. Nos. 6,376,829 and 6,433,353 describe a method and apparatus for detecting front surface irregularities in a glass plate. More specifically, the method described involves irradiating a beam of light toward a surface of a transparent plate at an angle of incidence between 86° and 89°, or 60° to 89° after such light beam is polarized (“P” or “S”-polarized) by a polarizing element between the light beam source and the transparent plate. A reflected image from a front surface of the transparent plate is then projected on a screen, is inspected by one of several possible methods, whereby density signals said to be representative of the reflected image are analyzed to calculate the irregularities present on the surface of the transparent plate.
U.S. Pat. No. 6,100,990 describes a method of determining reflective optical quality of a reflective product including reflecting a first gray scale pattern off the product; obtaining a first image of the first pattern with an image pickup device after the first pattern has reflected off the product; and determining optical quality of the product based on data obtained from the first image.
U.S. Patent Application Publication No. US2006/0050284A1 describes a process for scanning a surface of a substrate, which process consists in taking at least one reflected image of at least one test pattern on the substrate surface and extracting by digital processing, local phases in two directions. Variations in local slopes are calculated by digital processing from the local phases in order to deduce therefrom variations in curvature or variations in altitude of the substrate surface.
JP61223605A describes a method for inspecting the surface shape of an object by illuminating a specular surface and an inspecting plate by a light source which is arranged above the object to be inspected. Lines are recorded at a certain pitch on the lower face of the object. The light from the plate is made incident on the surface of the object through optical path A. The light from the plate is reflected on the surface of the object and follows optical path B to reach a lens. The lens forms the image of the stripe pattern recorded on the plate on the image receiving face of an image pickup device. The image pickup device converts this image to a video signal and sends it to a picture display device and a picture process or the strip pattern for the flat surface is stored in the processor, and this pattern is compared with the inputted stripe pattern, and the substrate surface is judged to be rugged if the patterns are different from each other.
WO99/34301 describes a three dimensional surface contouring method based on the full-field fringe projection technique. More particularly, a digital video projection system is used to project digitally created fringe patterns onto an object. The fringe pattern as distorted by the geometry of the object surface is then captured by a high resolution CCD camera. To increase contouring resolution, purely software-based digital phase shifting technique is used, which is said to eliminate the need for accurate positioning systems in the traditional phase shifting methods. The surface is reconstructed by applying phase wrapping and unwrapping algorithms.